The invention relates to an interconnection between different types of communications networks.
Mobile communication systems generally refer to different telecommunication systems which enable personal wireless data transmission while subscribers roam in the system area. A typical mobile communication system is a Public Land Mobile Network (PLMN). First-generation mobile communication systems were analogue systems where speech or data was transferred in an analogue form similarly as in conventional public switched telephone networks. An example of a first-generation system is the Nordic Mobile Telephone (NMT). In second-generation mobile systems, such as the Global System for Mobile Communication (GSM), speech and data are transmitted in a digital form. In addition to conventional speech transmission, digital mobile communication systems provide a plurality of other services: short messages, facsimile, data transmission, etc.
Currently under development are third-generation mobile communication systems, such as the Universal Mobile Communication System (UMTS) and the Future Public Land Mobile Telecommunication System (FPLMTS), which was later renamed as the International Mobile Telecommunication 2000 (IMT-2000). The UMTS is being standardized by the European Telecommunication Standards Institute (ETSI), whereas the International Telecommunication Union (ITU) standardizes the IMT-2000 system. These future systems are basically very similar. For example the UMTS, as all mobile communication systems, provides wireless data transmission services to mobile subscribers. The system supports roaming, which means that UMTS users can be reached and they can make calls anywhere as long as they are situated within the coverage area of the UMTS.
Transition to the use of third-generation mobile communication systems will take place gradually. At first, third-generation radio access networks will be used in connection with the network infrastructure of second-generation mobile communication systems. Such a hybrid system is illustrated in FIG. 1. A second-generation mobile services switching centre MSC is connected both to a second-generation radio access network, such as a GSM base station system BSS consisting of a base station controller BSC and base stations BTS, and to a third-generation radio access network consisting of, for example, a radio network controller RNC and base stations BS. In practice, there will be two different radio subsystems RSS, which share a common infrastructure on the network subsystem NSS level. Second-generation mobile stations MS (such as the GSM) communicate via the second-generation radio access network and third-generation mobile stations MS (such as the UMTS) communicate via the third-generation radio access network. Possible dual-mode phones (such as GSM/UMTS) are able to use either radio access network and to perform handovers between them. Subsequent development will lead to a situation where pure third-generation mobile communication networks exist in parallel with second-generation mobile systems or the aforementioned hybrid systems, as also illustrated in FIG. 1.
The interface between the 2G RAN (e.g. GSM BSS) and the 2G MSC (e.g. GSM MSC) is interface A, in which the transport/physical layer is a 64 kbits/s time division multiplexed (TDM) transmission channel, i.e. a PCM channel. The maximum data rate for a single traffic channel (one time slot) on the air interface is 9,6 kbits/s or 14,4 kbitls. In the HSCSD concept of the GSM system, a high-speed data signal is divided into separate data streams, which are then transmitted via N subchannels (N traffic channel time slots) at the radio interface. When the data streams have been divided, they are conveyed in the subchannels or substreams as if they were mutually independent until they are again combined in the IWF or the MS. However, logically these N subchannels or substreams belong to the same HSCSD connection, i.e. they form one HSCSD traffic channel. In the HSCSD concept the N subchannels are extended also over interface A up to the MSC.
In the UMTS architecture, the UMTS terrestrial radio access network, UTRAN, consists of a set of radio access networks RAN (also called radio network subsystem RNS) connected to the third generation (3G) MSC (or more generally, to a core network CN) through the interface Iu. Each RAN is responsible for the resources of its set of cells. For each connection between a mobile station MS and the UTRAN, one RAN is a serving RAN. A RAN consists of a radio network controller RNC and a multiplicity of base stations BS. The RNC is responsible for the handover decisions that require signaling to the MS. The base stations are connected to the RNC through the Iub interface.
On the interface Iu between the radio network controller RNC and the mobile switching centre MSC or the IWU, the transfer technique is the ATM (A synchronous Transfer Mode). The ATM transmission technique is a switching and multiplexing solution particularly relating to a data link layer (i.e. OSI layer 2, hereinafter referred to as an ATM layer. In the ATM data transmission, the end user""s data traffic is carried from a source to a destination through virtual connections. Data is transferred over switches of the network in standard-size packets called ATM cells. The ATM cell comprises a header, the main object of which is to identify a connection number for a sequence of cells forming a virtual channel for a particular call. A physical layer (i.e. OSI layer 1) may comprise several virtual paths multiplexed in the ATM layer. The ATM layer contains an ATM adaptation layer (ML) which enhances the service provided by the ATM layer to support functions required by the next highest layer. The AAL performs functions required by the user, control and management planes and supports the mapping between the ATM layer and the next higher layer. The functions performed in the AAL depend upon the higher layer requirements. At the moment, there are three different types of AAL: type 1 AAL (AAL1), type 2 AAL (AAL2) and type 5 AAL (AAL5).
The data rate at the UMTS air interface may be up to 2 Mbitls, which means that interface A between the IWU and 2G MSC must use the HSCSD concept in which a number of substreams is formed within each 64 kbits/s TDM channel. The 2G MSC (or more particularly, the associated interworking function IWF) naturally supports this feature, but in the case of the hybrid 2G/3G system, the HSCSD subchannels and protocols have to be implemented in the IWU merely because the standard interface A is required. Similar situation will be encountered on the interface between the 2G MSC and the 3G MSC after an inter-MSC handover. Therefore, in order to reduce the overall complexity, the idea of allowing xe2x80x9cmodestxe2x80x9d changes to the 2G interfaces when connecting them to the 3G network has been silently approved in the standardisation.
One proposed approach is a new protocol, A-TRAUxe2x80x2, on the interface between the 2G and 3G MSCs. The A-TRAUxe2x80x2 protocol is used on a plain 64 kbitls TDM channel without substreams, and thereby the complex protocol conversion relating to the substreams is avoided. The problem in this prior art approach is that it makes the protocol stacks rather complex by adding a completely new protocol to the 3G network and requiring that the new protocol is supported by the already existing 2G network elements. In other words, the number of different protocols increases. Further, there will still be two separate protocol legs, one (i.e. the Iu interface User Plane protocol, IuUP) on the Iu interface and the other (A-TRAUxe2x80x2) on the A interface. Finally, there will be an unnecessary protocol termination in the 3G-MSC in the case of a 2G-to-3G handover.
An object of the invention is to alleviate the problems described above.
This and other objects and advantages of the invention are achieved by means of communications systems, switching centres and an interworking unit as recited in the attached independent claims. Preferred embodiments on the invention are described in the dependent claims.
A first aspect of the invention is a communication system comprising
a first core network element connected to a first access network by means of a first access network-specific protocol stack used between said first core network element and a network element in said first access network,
a second core network node connected to a second access network by means of a second access network-specific protocol stack used between said""second core network element and a network element in said second access network,
the first core network element being arranged to support at least parts of the second protocol stack,
the first core network element being arranged to use at least parts of the second protocol stack towards the second core network element, and
the second core network element being adapted to extend the parts of the second protocol stack towards the first core network element.
According to a preferred embodiment of the invention, an access network-specific user-plane protocol which is already used and specified in the access network (such as 3G mobile communications network) and/or an ATM adaptation layer protocol which is specified and used between the user-plane protocol and an ATM layer in the radio access network is/are used also on the interface between radio access network and a lower generation switching centre (e.g. 2G mobile services switching centre). In other words, the same protocols already specified for another use are reused for interconnection purposes. Even though some changes are needed in the lower generation switching centre so that the reused protocols are supported, the invention avoids the need of a completely new protocol leg and the associated complexity. As the access network protocol(s) is (are) extended, less buffering and protocol conversions are needed. The conversions required due to the HSCSD in the pure interface A of the GSM system, for instance, are totally avoided, since the extended protocols of the 3G system are used instead. In its minimum, the functionality of the interworking can be limited to an ATM-TDM conversion. The number of protocol level buffers will be smaller, which results in smaller transmission delays. Further, as there is a smaller number of protocol legs to be initialized, the link set up will be faster.
Further, the same principle can be used both for interconnecting 2G and 3G MSCs in the case of intersystem handovers and for interconnecting a 3G radio access network (e.g. UTRAN) to an already existing 2G network core (e.g. GSM MSCs). As a result, the number of protocols to be supported in both networks is kept to a minimum, thus saving both implementation costs and processing (memory) capacity in network elements.
In an embodiment of the invention, only the access network-specific user-layer protocol is extended to the lower generation switching centre without the underlying ATM adaptation layer. In that case, the only protocol data units of the access network-specific user-layer protocol are mapped directly to the TDM transport layer channel. This reduces the overhead as the frame headers of the ATM adaptation layer are omitted.
According to another embodiment, in the case of a transparent data transmission, the switching centre is arranged to support only the ATM adaptation layer protocol on the interface between the radio access network and the lower generation switching centre. Only the ATM adaptation layer will be extended to the older generation switching centre by the interworking unit or switching center in the new generation network. The access network-specific user-plane protocol layer is in a mode wherein the user data is transparently transferred across the user-plane protocol layer between the ATM adaptation layer and upper layers. Therefore, the ATM adaptation layer PDUs can be relayed from the access network to the older generation switching centre over the TDM-type connection, and vice versa. The access network-specific user-plane protocol is terminated in the interworking unit or in the new generation switching centre. Also in this case unnecessary overhead is avoided.
According a still further embodiment of the invention, a frame synchronization of the new interface is performed by searching at least one control field having a constant value in a header of the protocol data unit received over the interface. The frame synchronization may be further assured by counting a check sum over the header and/or payload of the received protocol data unit and comparing the counted checksum with a respective checksum received in said respective protocol data unit.
The actual user data rate may be lower than the nominal data rate of the TDM channel(s). In such a case, rate adaptation may be performed between a lower user data rate and a higher nominal data rate of said interface by sending protocol data units with fill, sending protocol data units with data and fill, or sending protocol data units with repetition of data, and discarding the fill or repetition on a receiving side.